Recently, due to progresses made in digital techniques and improvements made in data compression techniques, there is much attention on optical disks for storing information (hereinafter also referred to as “contents”) such as music, movies, photographs and computer software. The optical disks include compact disks (CDs), digital versatile disks (DVDs) capable of recording approximately 7 times the amount of data recordable on a CD having the same diameter as the DVD. Optical disk apparatuses which use the optical disks as the information recording medium have become popular as the optical disks have become less inexpensive.
The optical disk apparatus records information on or erases information from a spiral track or concentric tracks on the optical disk by irradiating laser light on a recording layer of the optical disk, and reproduces information from the spiral track or concentric tracks based on a reflected light (returned beam) from the recording layer. Hence, the optical disk apparatus is provided with an optical pickup unit that emits the laser light and receives the returned beam.
Generally, the optical pickup unit includes an objective lens, an optical system that guides the laser light emitted from a light source to the recording layer and guides the returned beam to a predetermined light receiving position, and a photodetector that is arranged at the light receiving position. The photodetector outputs signals including not only reproduced information of data recorded in the recording layer, but also information (servo information) necessary to control the position of the objective lens.
The amount of information of the contents is increasing every year, and there are demands to further increase the recording capacity of the optical disk. As one means of increasing the recording capacity of the optical disk, information recording media having a plurality of recording layers and apparatuses which use such information recording media as the access target have been developed. For example, such information recording media and apparatuses which use such information recording media as the access target are proposed in Japanese Laid-Open Patent Applications No. 2001-52342, No. 2002-74679, No. 2003-91874, No. 8-147762, No. 11-96568 and No. 2002-334448.
In order to control the position of the objective lens with respect to the information recording medium having a plurality of recording layers so that a light spot is accurately formed at a target position, it is necessary to accurately discriminate the recording layer on which the light spot is formed. In addition, in order to achieve a high-speed access, the position of the objective lens must be controlled at a high speed. However, according to the information recording media and the apparatuses proposed in the Japanese Laid-Open Patent Applications No. 2001-52342, No. 2002-74679, No. 2003-91874, No. 8-147762, No. 11-96568 and No. 2002-334448 referred above, it sometimes takes time to discriminate the recording layer on which the light spot is formed, and the access time may become long.